Hitherto, a planar light circuit (PLC) formed on a substrate is used in the field of optical communications. A planar light circuit is used mainly as a silica-based material as the material of an optical waveguide. For example, a planar light circuit of an array waveguide grating (AWG) employing a silica-based material, a splitter, or the like is used as a backbone component in optical communications.
Recently, an attempt to realize a smaller system by mounting an active optical device and a passive optical device on a common PLC substrate is being made. For example, a novel optical device such as a wavelength-variable light source in which a compound semiconductor amplifier (SOC) is hybrid-packaged on a silica-based PLC is being developed.
However, as a demanded function is becoming more complicated and sophisticated, the device dimension and consumption power of the conventional planar optical circuit are increasing. Consequently, it is becoming difficult to improve the function or performance of an optical circuit using the conventional silica-based PLC.
Accordingly, a technique of forming an optical waveguide on an SOI (Silicon On Insulator) substrate using a silicon microfabrication technique such as a silicon wire waveguide or a photonic crystal (PC) is being studied. Development of a small backbone component characterized by low power consumption is being examined using the silicon microfabrication technique.
A silicon wire waveguide includes, for example, a channel-type optical waveguide in which a core layer is disposed on a lower cladding layer and a rib-type optical waveguide in which a slab layer is disposed on a lower cladding layer and a core layer is formed on the slab layer. On the core layer or the rib layer, an upper cladding layer is disposed. The optical waveguides have different characteristics.
As the channel-type optical waveguide, a bending optical waveguide having low optical loss can be formed. For example, an optical waveguide having a bending radius of a few microns to about ten microns can be formed using a channel-type optical waveguide while suppressing optical loss. On the other hand, in the channel-type optical waveguide, the influence of a change in structure parameters such as width and thickness exerted on optical characteristics such propagation loss and effective refractive index is large, so that the allowable range of dimension precision in manufacture is small. Such a characteristic may become an issue when manufacturing a resonator or a filter using the channel-type optical waveguide. Consequently, there is a case that the channel-type optical waveguide is requested to have high manufacturing precision.
In the rib-type optical waveguide, as compared with the channel-type optical waveguide, the influence of a change in structure parameters exerted on optical characteristics such as propagation loss and effective refractive index is smaller, so that dimension precision demanded in manufacture is lessened. On the other hand, in the rib-type optical waveguide, the optical loss of the bending optical waveguide is larger than that in the channel-type optical waveguide. Consequently, there is a case that it is unsuitable to form a bending optical waveguide having a small radius of curvature.
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